Secondary radars interrogate aircraft fitted with transponders, the aircraft being detected by radars which, under these circumstances, are termed primary radars. However, although the identification system according to my invention is used mainly with aircraft, it is not strictly limited to these and may be applied to other objects carrying transponders.
Systems for identifying objects carrying transponders are well known and the way in which they operate will be briefly reviewed in what follows.
A fixed transmitter, which as part of a secondary radar is situated on the ground, interrogates in a directional fashion an aircraft equipped with a transponder. The interrogation signals specify a "mode" in which the aircraft should respond. These modes are standardized and are known as 1, 2, 3/A, B, C or D under the International Convention adopted for civil aviation.
The transponder being interrogated, assuming it to be in the same mode as that indicated by the interrogation, transmits, omnidirectionally, to the ground a binary message which represents a code identifying the aircraft and/or its destination. The code, which is laid down in flight procedures, is known at the ground end. When the message transmitted by the aircraft is received, the ground station recognizes the code laid down in flight procedures and identifies the aircraft as friendly; it also learns the destination for which the aircraft is headed and what it is doing at the time of interrogation. It is clear that a number of interrogation and response cycles may take place while the aircraft is in the interrogation beam emitted by the antenna of the secondary radar. In the case here considered the response is formed by a code word of 14 bits, the first and last bit positions being occupied by framing pulses. Since the secondary radar may be used by both civil and military authorities, a security problem may arise for the military authorities when they wish to prevent an enemy aircraft equipped with a transponder from picking up messages transmitted by a friendly aircraft and discovering the answering code which is used.
Such a relatively simple system is easy to jam, easy to deceive, unable to respond to two interrogations coming from two separate radars, and incapable of responding an incomplete interrogation signal. Furthermore, it is able to transmit only a single flight parameter, namely altitude (mode C).